Which is the exact reason that I suggested 5 Alkaline (5X 1.3 = 6.5V) And why I suggested that 5 NI-MH or NI-CD batteries would work because the NI-MH or NI-CD batteries have a reasonably "flat" discharge curve @ ~ 1.2V for about 80% of their effective charge life. I "Did" batteries for 15 or 20 years as design subjects and as actual parts, 95% of my work from 1998 to 2004 or 5 was on solar powered "Process" controllers for the irrigation industry. They were radio controlled valve controllers that operated a bi-stable or latching solenoid and charged accurately an SLA battery by voltage and tailored to temperature as the optimal charge value is different as the temperature changes being worst at low temps...
Bob